This invention relates to an exhaust gas purifying apparatus for use in an internal combustion engine having a catalytic trap provided in the engine exhaust system.
Internal combustion engines discharge undesirable exhaust gas constituents, such as unburned carbon particles, to the atmosphere. The emission from an engine tends to increase at low- or intermediate-speed and high-load conditions where the engine requires a mixture having a richer air-fuel ratio. This is true particularly for diesel engines.
It has been proposed to purify such exhaust gas constituents by employing a catalytic trap located in the engine exhaust system for collecting the exhaust particles discharged thereto from the engine and burning the collected exhaust particles under catalytic action. However, the exhaust particle burning capacity of the catalytic trap is dependent greatly on the engine operating conditions and it becomes smaller than the amount of the exhaust particles collected in the catalytic trap particularly in a certain engine operating range, as described later in greater detail. In such an engine operating range. the catalytic trap gets clogged with the exhaust particles accumulated therein to increase the engine exhaust pressure, causing engine output power reduction and fuel economy loss.